volume: add "readBufSize" option to customize read optimization (#3702)

* simplify a bit

* feat: volume: add "readBufSize" option to customize read optimization

* refactor : redbufSIze -> readBufferSize

* simplify a bit

* simplify a bit

weed/storage/store.go

@@ -44,6 +44,10 @@ type ReadOption struct {
 	//  * read requests should complete asap, not blocking other requests.
 	//  * write requests may see high latency when downloading large files.
 	HasSlowRead bool
+
+	// increasing ReadBufferSize can reduce the number of get locks times and shorten read P99 latency.
+	// but will increase memory usage a bit. Use with hasSlowRead normally.
+	ReadBufferSize int
 }
 
 /*

weed/storage/volume_read.go

@@ -136,7 +136,7 @@ func (v *Volume) readNeedleDataInto(n *needle.Needle, readOption *ReadOption, wr
 		actualOffset += int64(MaxPossibleVolumeSize)
 	}
 
-	buf := mem.Allocate(min(1024*1024, int(size)))
+	buf := mem.Allocate(min(readOption.ReadBufferSize, int(size)))
 	defer mem.Free(buf)
 
 	// read needle data